Ukrainian President Volodymyr Zelenskiy has postponed all foreign travel as the battlefield situation continued to deteriorate and Kyiv said Russian infantry had entered the border town of Vovchansk in the Kharkiv region.
The capture of the northeast town 5km from the border would be Russia's most significant gain since it launched an incursion into the Kharkiv region on Friday, opening a new front and forcing Kyiv to rush in reinforcements.
The assault keeps Ukraine's stretched and depleted forces off balance ahead of what Zelenskiy has said could be a big Russian offensive in the coming weeks. Moscow has been slowly making ground in the east for months.
"The situation is extremely difficult. The enemy is taking positions on the streets of the town of Vovchansk," Oleksiy Kharkivskiy, Vovchansk's patrol police chief, said on Facebook.
Ukrainian troops pulled back to "more advantageous" positions in two areas of the Kharkiv region, including the Vovchansk area, the military said late on Tuesday.
It said the decision was "a consequence of enemy fire and storming action" and taken "to preserve the lives of our servicemen and avoid losses".
Defence ministry spokesman Dmytro Lazutkin said "some" Russian infantry groups had entered the town, which military analysts reckon Moscow needs to capture to continue its offensive thrust in that direction.
Kyiv's forces were trying to prevent Russia from building up troops and military hardware in Vovchansk's north, the military said. The Russians were trying to regroup and dig in and had not taken any more "active" action on Wednesday, it added.
Police remained in Vovchansk and were continuing to evacuate people, Kharkivskiy said. Nearly 8000 people have been evacuated from Vovchansk and border areas since Friday's assault.
Zelenskiy has postponed all his foreign travel, his spokesman Sergiy Nykyforov said, after the Ukrainian leader held a daily conference call with senior military figures to discuss the situation in Kharkiv region and the supply of weapons.
"Volodymyr Zelenskiy has instructed that all international events scheduled for the coming days be postponed and new dates coordinated," Nykyforov wrote on Facebook.
Ukraine is trying to snuff out the assault in the Kharkiv region, while holding the line against Moscow's main thrust in the eastern Donbas region and guarding against potential new border incursions.
Ukraine's top military spy has warned that Russia had small groups of forces located to the north of Kharkiv region along the Sumy region.
Kyiv says the Russian assault into the northeast does not present an imminent threat to the region's city of Kharkiv, Ukraine's second largest, that is home to 1.3 million people.
Ukraine's shortage of troops is compounded by months of delayed deliveries of weapons, in particular from the United States where Congress took six months to approve a major aid package.
The deteriorating situation in Kharkiv region coincided with a visit to Kyiv by US Secretary of State Antony Blinken who said some of the US aid had arrived and that more was on the way. That, he said, would "make a real difference".
"I know this is a really, really difficult time. Your soldiers, your citizens, particularly in the northeast in Kharkiv, are suffering tremendously," he said.
"But they need to know, you need to know, the United States is with you, so much of the world is with you. And they’re fighting not just for a free Ukraine but for the free world, and the free world is with you too."
